Fluxes and sources of particulate organic carbon in the Ganga‐Brahmaputra river system

Sources and fluxes of particulate organic carbon (OC) in the Ganga‐Brahmaputra river system were estimated from 13 C/ 12 C ratios of bedrocks, soils, bank and suspended river sediments from a Himalayan watershed (Narayani, Nepal), and the Bangladesh floodplain. In the watershed of the Narayani, OC δ 13 C values for bank and suspended sediments have similar values and narrow ranges, with a mean of about −24.2‰ at the base of the Himalayas. On the Bangladesh floodplain, bank and suspended sediments are enriched in 13 C (δ 13 C about −22.5‰) relative to those collected at the base of the Himalayas. The OC exported at the base of the Himalayan range is estimated to include about 15% from the 13 C enriched C 4 biomass while the C 4 contribution is about 25% of the OC exported by the Ganga and Brahmaputra. A calculated total OC flux of 0.65 × 10 12 molC yr −1 is exported to the ocean or trapped in the plain. In sediments of the Ganga and Brahmaputra, the clay‐size fraction is depleted in 13 C (<2.6‰) relative to the bulk sediment. Possible explanations are that either the organic matter associated with clays is largely inherited from the Himalayan watersheds, or, and considered more likely, the clays and coarser sediments sample different OC pools, possibly with different ages (the growth of C 4 crops has dramatically increased since the mid twentieth century), on the floodplain. The OC budget of the actual Ganga‐Brahmaputra system is broadly comparable to that derived from the Quaternary sediments of the Bengal fan that represents about 10% of the global OC contribution to the continental margins.